Conventional rotation shafts provided in various drive force transmission mechanisms are rotatably supported via bearings. For such rotation shafts, so-called ball bearings are used in order to improve rotation performance of the rotation shafts. Such ball bearings include multiple rolling members (rolling elements) circumferentially disposed between inner and outer rings.
Such ball bearings (hereunder referred to as “bearings”) are used as support members for rotation shafts of drive force transmission mechanisms in various drive units, and some drive units are desired to prevent foreign substances such as dusts and water from penetrating through the bearings into the units. Moreover, when foreign substances enter into the bearings themselves, rotation performance of such shafts could be deteriorated and noise could be caused.
In order to prevent such a problem, a magnetic fluid sealed bearing that includes a magnet and a retaining plate (a polar plate) between an inner ring and outer ring of the bearing to form a magnetic circuit. Magnetic fluid is retained by the magnetic circuit and thereby the inside of the bearing is tightly sealed. Japanese Patent Application Publication No. S57-033222 (the “'222 Publication”) discloses such a magnetic fluid sealed bearing. In the magnetic fluid sealed bearing, the magnet and the polar plate are fixed to the outer ring so as to create a clearance between the inner ring and the outer ring, and magnetic fluid is retained within the clearance. In this manner the inside of the bearing is sealed.
Because the clearance is sealed with the magnetic fluid in the above-described magnetic fluid sealed bearing, it is possible to enhance the sealing of the inner portion of the bearing without reducing the rotation performance compared to other sealing structures such as rubber. However, there is a possibility that a sufficient sealing cannot be provided at the side where the magnet and retaining plate are fixed (the outer ring side).
For this reason, the '222 Publication discloses a magnetic fluid sealed bearing in which the fixed side of the magnet and the retaining plate (press-fitted side) is also filled with the magnetic fluid in order to enhance the sealing of the inner portion of the bearing. More specifically, the magnet attached to the retaining plate is magnetized such that the magnetic poles point in the axial direction so that the magnetic circuits are formed on the inner and outer ring sides symmetrically with the axial direction. Sealing films made of the magnetic fluid are formed on the outer and inner ring sides by retaining the magnetic fluid in each of the magnetic circuits and the sealing of the inner portion can be enhanced.
As another means to handle the problem, an elastic seal member is provided on a portion of the periphery of a rotation shaft proximate to a bearing to shut out water and dusts from the bearing, however, the rotation performance of the rotation shaft may be degraded due to contact pressure caused by the elastic seal member.
A bearing with a magnetic fluid sealing using magnetic fluid (hereunder referred to as a magnetic fluid sealed bearing) is known for preventing foreign substances from penetrating through the bearing without degrading the rotation performance of the rotation shaft. For instance, the '222 Publication discloses a magnetic fluid seal (magnetic sealing mechanism) with a ball bearing that includes a rolling member between the outer and inner rings. A magnetic body is interposed between the outer ring and the inner ring that rotate relatively to each other and one side of the magnetic body is fixed and other side of the magnetic body is provided with magnetic fluid in the clearance.
The magnetic fluid seal includes a magnet forming a magnetic circuit with the inner ring or the outer ring, a retaining plate retaining the magnet, and a magnetic fluid retained between the inner ring or the outer ring and the retaining plate. The magnetic fluid is retained by the magnetic circuit and thereby the inner portion of the bearing body is sealed. More specifically, magnetic body is disposed between the inner ring and the outer ring, one side of the magnetic body is fixed, and a magnetic fluid is provided in a clearance on the other side of the magnetic body thereby the rolling member is hermetically sealed. In this way, penetration of foreign substances into the rolling member section can be prevented.
The magnetic fluid sealed bearings disclosed in the '222 Publication and Japanese Patent Application Publication No 2013-228044 include the rolling member commonly made of a magnetic material such as SUS440C. The magnet of the magnetic fluid sealing forms a magnetic circuit in a small area formed between the magnet and the outer or inner ring disposed closely to the magnet. A strong magnetic force generated from the magnetic circuit acts on the rolling member and consequently a magnetic attraction force between the rolling member and the inner or outer ring is increased. This works as resistance that hampers the rotation of the rolling member. In other words, the rolling member is attracted toward the inner or outer ring by the magnetic force and this hampers the smooth rotation of the rolling member (increases the rotational torque).
As described above, the magnetic fluid sealed bearing can improve the sealing quality of the bearing compared with conventional bearing but still have the drawback that the magnetic circuit generated by the magnet largely affects the rolling member and the rotational torque is increased (reducing the ease of rotation).
Solving this disadvantage related to the magnetic fluid sealed bearing is important especially for the bearing function in a rotational driving section of fishing reels that are required a high rotation performance (a high free-rotation performance with a low torque) in addition to a water and dust prevention feature.